Force cast features label values to string

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#### I'm submitting a ...
- feature request

#### What is the current behavior?
https://github.com/allure-framework/allure-python/blob/50cb0ec0f388928eabf1fc5af92a009851d34965/allure-python-commons/src/_allure.py#L45-L46

Features values set as is.

#### What is the expected behavior?

Cast `features` to string

#### What is the motivation / use case for changing the behavior?

For easily cast to feature name from dataclasses

Its look better
```python
@allure.feature(Features.some_feature1, Features.some_feature2)
```
vs
```python
@allure.feature(str(Features.some_feature1), str(Features.some_feature2))
```

#### Please tell us about your environment:

- Allure version: 2.1.0
- Test framework: pytest@6.2.4
- Allure adaptor: allure-pytest@2.9.43

#### Other information

[//]: # (
. e.g. detailed explanation, stacktraces, related issues, suggestions
. how to fix, links for us to have more context, eg. Stackoverflow, Gitter etc
)

Start at allure-python-commons/src/_allure.py around lines 45-46, where feature values are currently kept as supplied. Trace the feature decorator entry point and its existing tests, then ensure feature values are converted to strings so dataclass members can be passed directly; done means the resulting feature names are string values.

Written by the indexing model from the issue text.